Columnar: Fix VACUUM for empty tables

src/backend/columnar/cstore_metadata_tables.c

@@ -607,13 +607,23 @@ StripesForRelfilenode(RelFileNode relfilenode)
 /*
  * GetHighestUsedAddress returns the highest used address for the given
  * relfilenode across all active and inactive transactions.
+ *
+ * This is used by truncate stage of VACUUM, and VACUUM can be called
+ * for empty tables. So this doesn't throw errors for empty tables and
+ * returns 0.
  */
 uint64
 GetHighestUsedAddress(RelFileNode relfilenode)
 {
 	uint64 highestUsedAddress = 0;
 	uint64 highestUsedId = 0;
-	ColumnarMetapage *metapage = ReadMetapage(relfilenode, false);
+	ColumnarMetapage *metapage = ReadMetapage(relfilenode, true);
+
+	/* empty data file? */
+	if (metapage == NULL)
+	{
+		return 0;
+	}
 
 	GetHighestUsedAddressAndId(metapage->storageId, &highestUsedAddress, &highestUsedId);
 

src/backend/columnar/cstore_tableam.c

@@ -743,7 +743,8 @@ LogRelationStats(Relation rel, int elevel)
 	appendStringInfo(infoBuf,
 					 "total row count: %ld, stripe count: %d, "
 					 "average rows per stripe: %ld\n",
-					 tupleCount, stripeCount, tupleCount / stripeCount);
+					 tupleCount, stripeCount,
+					 stripeCount ? tupleCount / stripeCount : 0);
 	appendStringInfo(infoBuf,
 					 "block count: %ld"
 					 ", containing data for dropped columns: %ld",
@@ -817,7 +818,11 @@ TruncateCStore(Relation rel, int elevel)
 	SmgrAddr highestPhysicalAddress =
 		logical_to_smgr(GetHighestUsedAddress(rel->rd_node));
 
-	BlockNumber new_rel_pages = highestPhysicalAddress.blockno + 1;
+	/*
+	 * Unlock and return if truncation won't reduce data file's size.
+	 */
+	BlockNumber new_rel_pages = Min(old_rel_pages,
+									highestPhysicalAddress.blockno + 1);
 	if (new_rel_pages == old_rel_pages)
 	{
 		UnlockRelation(rel, AccessExclusiveLock);

src/test/regress/columnar_am_schedule

@@ -8,6 +8,7 @@ test: am_query
 test: am_analyze
 test: am_data_types
 test: am_drop
+test: am_empty
 test: am_insert
 test: am_copyto
 test: am_alter

src/test/regress/expected/am_empty.out

@@ -0,0 +1,97 @@
+--
+-- Test different operations on empty columnar tables.
+--
+SET citus.compression to 'none';
+create table t_uncompressed(a int) using columnar;
+create table t_compressed(a int) using columnar;
+-- set options
+SELECT alter_columnar_table_set('t_compressed', compression => 'pglz');
+ alter_columnar_table_set
+---------------------------------------------------------------------
+
+(1 row)
+
+SELECT alter_columnar_table_set('t_compressed', stripe_row_count => 100);
+ alter_columnar_table_set
+---------------------------------------------------------------------
+
+(1 row)
+
+SELECT alter_columnar_table_set('t_compressed', block_row_count => 100);
+ alter_columnar_table_set
+---------------------------------------------------------------------
+
+(1 row)
+
+SELECT * FROM cstore.options WHERE regclass = 't_compressed'::regclass;
+   regclass   | block_row_count | stripe_row_count | compression
+---------------------------------------------------------------------
+ t_compressed |             100 |              100 | pglz
+(1 row)
+
+-- select
+select * from t_uncompressed;
+ a
+---------------------------------------------------------------------
+(0 rows)
+
+select count(*) from t_uncompressed;
+ count
+---------------------------------------------------------------------
+     0
+(1 row)
+
+select * from t_compressed;
+ a
+---------------------------------------------------------------------
+(0 rows)
+
+select count(*) from t_compressed;
+ count
+---------------------------------------------------------------------
+     0
+(1 row)
+
+-- explain
+explain (costs off, summary off, timing off) select * from t_uncompressed;
+                 QUERY PLAN
+---------------------------------------------------------------------
+ Custom Scan (CStoreScan) on t_uncompressed
+(1 row)
+
+explain (costs off, summary off, timing off) select * from t_compressed;
+                QUERY PLAN
+---------------------------------------------------------------------
+ Custom Scan (CStoreScan) on t_compressed
+(1 row)
+
+-- vacuum
+vacuum verbose t_compressed;
+INFO:  statistics for "t_compressed":
+storage id: -1
+total file size: 0, total data size: 0
+total row count: 0, stripe count: 0, average rows per stripe: 0
+block count: 0, containing data for dropped columns: 0, none compressed: 0, pglz compressed: 0
+
+vacuum verbose t_uncompressed;
+INFO:  statistics for "t_uncompressed":
+storage id: -1
+total file size: 0, total data size: 0
+total row count: 0, stripe count: 0, average rows per stripe: 0
+block count: 0, containing data for dropped columns: 0, none compressed: 0, pglz compressed: 0
+
+-- vacuum full
+vacuum full t_compressed;
+vacuum full t_uncompressed;
+-- analyze
+analyze t_uncompressed;
+analyze t_compressed;
+-- truncate
+truncate t_uncompressed;
+truncate t_compressed;
+-- alter type
+alter table t_uncompressed alter column a type text;
+alter table t_compressed alter column a type text;
+-- drop
+drop table t_compressed;
+drop table t_uncompressed;

src/test/regress/sql/am_empty.sql

@@ -0,0 +1,48 @@
+--
+-- Test different operations on empty columnar tables.
+--
+
+SET citus.compression to 'none';
+create table t_uncompressed(a int) using columnar;
+create table t_compressed(a int) using columnar;
+
+-- set options
+SELECT alter_columnar_table_set('t_compressed', compression => 'pglz');
+SELECT alter_columnar_table_set('t_compressed', stripe_row_count => 100);
+SELECT alter_columnar_table_set('t_compressed', block_row_count => 100);
+
+SELECT * FROM cstore.options WHERE regclass = 't_compressed'::regclass;
+
+-- select
+select * from t_uncompressed;
+select count(*) from t_uncompressed;
+select * from t_compressed;
+select count(*) from t_compressed;
+
+-- explain
+explain (costs off, summary off, timing off) select * from t_uncompressed;
+explain (costs off, summary off, timing off) select * from t_compressed;
+
+-- vacuum
+vacuum verbose t_compressed;
+vacuum verbose t_uncompressed;
+
+-- vacuum full
+vacuum full t_compressed;
+vacuum full t_uncompressed;
+
+-- analyze
+analyze t_uncompressed;
+analyze t_compressed;
+
+-- truncate
+truncate t_uncompressed;
+truncate t_compressed;
+
+-- alter type
+alter table t_uncompressed alter column a type text;
+alter table t_compressed alter column a type text;
+
+-- drop
+drop table t_compressed;
+drop table t_uncompressed;